Electrokinetic properties of methylated quartz capillaries

Electrokinetic (ζ)-potentials of methylated (trimethylchlorosilane) quartz capillaries (5–6 μm in radius) were determined in 10−4 M KCl solution. Over the course of time, the absolute values of the ζ-potential decrease, as a result of the formation of small bubbles on the rough methylated surface, generated from the flowing, nitrogen gas-saturated solution. This decrease is attributed to screening of a part of the solid surface. After the passage of time, a sharp increase in the ζ-potentials was observed, as the pressure was increased and the initial potential values were recovered. Sometimes, oscillations in the ζ-potentials were observed. This behaviour was explained by detachment of bubbles from the methylated surfaces by the flowing solution. Addition of non-ionic surfactant, which made the methylated surface hydrophilic, decreased the measured ζ-potentials. This was attributed to suppression of water slippage, an effect known to occur for hydrophobic solid surfaces. A mixed mechanism of charge formation is characteristic for these methylated quartz surfaces and is connected with presence of hydrophobic and hydrophilic areas. The ratio between these areas controls both the formation of surface charge as well as the contact angles.